Under threat of perjury, she was advised to tell the naked truth on the stand.

Origin:

This phrase comes from an old fable: "Truth and Falsehood went swimming.
Falsehood stole the clothes that Truth had left on the river bank, but Truth refused to wear Falsehood's clothes and went naked."

Something that is popular but soon becomes unfashionable or ceases to be novel.

Example:

Origin:

An old proverb states "A wonder lasts nine days, and then the puppy's eyes are open", alluding to dogs which, like cats, are born blind. The public are reckoned to be blind initially, but then their eyes are opened.

Alternative: The Latin word "novena" is a term used in the Roman Catholic Church for a devotion lasting for nine consecutive days. Once completed, a new one may be instituted and the other forgotten.

They say being rich is
not all it's cracked up to be. But then again, neither
is being poor.

Origin:

"Crack" can be used as
a noun meaning a statement or comment. Consider the term "wise crack".

As a verb "crack", dating back to the 15th century, was to praise or
boast. If you "cracked up" something you sang its praises.
Therefore something that is "not all it's cracked up to be" is something
that is not as good as you were told.

The cat in this phrase is of the cat o'nine tails type. The nine-thronged whip was used as punishment at sea. Because space was at a premium below decks, there was not enough room to wield the whip. As a result, the whipping always took place on deck.

Alternative: Evidence against the above origin comes from the fact that the expression was in use in the 1500s and the cat o'nine tails was not invented until the mid 1600s. The saying may involve felines, since there used to be a "sport" of swinging cats as targets for archers. This was either by their tails, in a sack, or in a leather bottle. Shakespeare, in "Much Ado About Nothing" (I,i) uses the phrase "hang me in a bottle like a cat".

In the early days of boxing, there was no bell
to signal the beginning of a round. Instead, the referee whould scratch a line on the ground
between fighters, and the round began when both men stepped over it. When a boxer couldn't
cross the line to keep a match going, people said he was not "up to the scratch."